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On the Role of Reversible and Irreversible Adsorption Hydrogen in the Dehydrogenation and Reforming Reactions
Guni, L a ul. (Editors), New Fronriers in Catalysis Proceedings of the 10th International Congress on Catalysis, 19-24 July, 1992,Budapest, Hungary 8 19!B Elsevier Science Publishers B.V.All rights reserved
ON TEE ROLE OF REVERSIBLE AND IRREVERSIBLE ADSORPTION HYDROGEN IN THE DEHYDROGENATION AND REFORMING REACTIONS Y. Sun, S. Chen andS. Peng
Instituteof Coal Chemistry,Chinese Academy of Sciences,P.O.Box 165, Taiyuan,Shanxi 030001, China
The conversion of hydrocarbon over Pt catalysts is very important in petroleum processing. Their catalytic performance, particularly product selectivity, depends strongly on the pressure of H2 and its sorption behavior. Thus, much attention has been paid to investigate the effect of H2 adsorption on hydrocarbon conversion over supported Pt catalysts[l] . However, only irreversible adsorption hydrogen has been considered in almost all conclusions. Under a real reaction condition, in our opinion, irreversible and reversible adsorption species co-exist on catalyst surface. The phenomenon has been observed in our experiments[l-3]. Furthermore, it is found that the irreversible and reversible species could play different roles in a catalytic reaction. The reversible adsorption species refers to a type of adsorbed species existing on the catalyst surface at reaction temperature when there is its gas phase pressure in the system and once the pressure is removed by evacuation or purging. it would be completely desorbed, while the irreversible adsorption species can not be removed from the catalyst surface. Therefore, it is necessary t o investigate deeply the roles of the two adsorbed species. The present paper is fucosed on the roles of irreversible and reversible adsorbed hydrogen in dehydrogenation and reforming reaction over supported Pt catalysts.
Catalysts containing 0.5%(wt) platinum were prepared by impregnation of
11 -A1203 of surface area of 239m2/g in HzPtCls solution. The composition
and pretreatment of catalysts were given in Table 1. All catalysts were reduced at 450.c in flowing H2. Both static chemisorption by Chemisorb 2800 (Micromeritics Co., USA) and dynamic adsorption by pulse and FTPS methodl41 was used to measure the hydrogen isobars at different H2 pressure and temperature from 35.c to 550.c. Here the reversible and irreversible uptake of H2 was represented
by Hr/Pt and Hi/Pt respectively, and the fraction of reversibly and irreversibly adsorbed hydrogen in total uptake was symbolized by Fr and Fi respectively. Table 1 Composition and pretreatment conditions catalysts Ga/Pt atom ratio Cl/Pt atom ratio PH value of soiution Calcination temp. (%I
AH1
AH4 ---- AH21 ---- AH2 --- ----
G11 1.04 1.41 2.28 1.32 1.24 2.17 2.54 1.94 1.94 1.15 2.54 450 120 450 450 450
GP2 CP3 2.08 2.89 1.72 ---2.54 2.54 450 450
GP4 3.78 1.46 2.54 450
The data of the coversion of n-hexane at 360.c and 450'c and the dehydrogenation of cyclohexane at 300% in the pressure of H2 were determined with 150 mg of catalyst in a continuous flow microreactor. The feed rate of H2 and n-hexane with volume ratio 1OOO:l was 60 ml/min, and for dehydrogenation, the mole ratio of Hz and cyclohexane is 1O:l. The reliable data were taken after reacting 10 hours. The selectivity of isomerization, dehydrocycl ization and hydrocracking was represented by Si, Sb and Sh respectively. The deactivation amount was given by the difference AX of conversions at initial time Xo and after running 10 hours Xi00
3.RESULT AND DISCUSSION 3.1.The Effect of the Irreversible Hydrogen on the Dehydrogenation of Cyc 1ohexane At 300 'c and atmospheric pressure, benzene was the unique product of cyclohexane dehydrogenation over supported Pt catalysts, and after catalyst running for ten hours, no deactivation was 0bserved.h this case, it was very interesting to note that there was linear dependence between the conversion of cyclohexane and irreversible uptake of hydrogen Fi, which could be expressed empirically as follows: X=1.89-2.44Fi (1) This equation revealed that the irreversibly adsorbed hydrogen on the catalyst surface at the reaction temperature suppressed the dehydrogenation of cyclohexane, i.e., with the increase of the fraction of irreversible hydrogen, catalyst activity showed a 1 inear decrease. Furthermore, the relationship was independent of catalyst preparation. It could, therefore, be reasoned that although the dehydrogenation could also take place on the modified catalyst surface by the irreversible hydrogen, the rate was slower than that on the surfase of fresh catalyst. 3.2. The Effect of Reversible and Irreversible Hydrogen on Conversion of N-hexane For the reaction of n-hexane over Pt catalyst in the presence of hydrogen. the product .was quite sensitive to the reaction temperature. And
2397 therefore, 360'c and 450'c of temperature were selected to study the isomerization and the aromatization of n-hexane. Surprisingly, it was also found that there existed the linear relationships between the reversible hydrogen and the product selectivities and catalyst deactivation amount. They could be empirically expressed by the following equations: at 360.c Si=2.68Fi-. 8354=1.845-2.68Fr r=O.996 (2) r=o.991 (3) 1202-8.3Hr/Pt at 450.c Sb=l.67Fi-0.0334=1.647-1.67Fr r=O.993 (4) r=O.999 (5) SitSh=2.17Fr-1.0472 (6) r=O.996 32-15.OHr/Pt It could be seen from these equations that adsorption type had a crucial effect on the product distribution of n-hexane conversion over supported Pt catalyst, i.e., the increase in the relative amount of irreversible hydrogen promoted the isomerization at 360.c and the aromatization at 450*c,while that in the relative amount of reversible hydrogen did the hydrocracking and isomerization at 450.c ; meanwhile, the reversible hydrogen could protect the deactivation of catalysts. These results demonstrate that not only irreversible but also reversible adsorption species did play important roles in a heterogeneous catalytic reaction, and further indicate that the reaction could take place on the catalyst surface modified by the irreversibly adsorbed hydrogen. This implied that the irreversible species were not only an active reactant but also a modifier of catalyst surface and even could induce reaction or active catalylic sites, while the reversible species mainly act as an active reactant or/and precursor of irreversible species. Furthermore, the negative intercept in equations (2), (4) and (5) implied that the reaction could occur only when there was certcain relative amount of irreversible hydrogen on the surface of the catalyst.and the negative slope in equation (3) and (6) suggested that the supported Pt catalyst would be not deactivated if there was sufficient amount of reversibly adsorbed hydogen on the surface of catalysts.
nX=O.
nX=O.
4. CONCLUSIONS
Not only irreversibly but also reversibly adsorbed hydrogen at the reaction temperature plays important roles in catalytic conversian of hydrocarbon over supported Pt catalyst. The irreversible hydrogen could also induce the reaction o r even modify activate catalytic sites, while the reversible hydrogen acts an active reactant or/and precursor of irreversible species.
The financial support of the National Natural Science Foundation of China is gratefully acknowledged.
2398
REFERENCES: Z. Paol. in Hydrogen Effect in Catalysis - Fund. Prac. Appl(Z. Paol and P.C.Menon, Eds.1 , Marcel Dekker Inc. New York, 1988. 2 . Y.H. Sun, Study on the Influence of Preparation Parameters on the Sorption of H2 and Catalytic Comersion of Cs Hydrocarbons over Supported Pt catalyst, Ph. Dr. Thesis, Institute of Coal Chemistry, 1989. 3 . G.M. Lou, Study on the Effect of Reversibly and Hydrogen on Selective Hydrogenation of Unsaturated Hydrocarbon, Ph. Dr. Thesis, Institute of Coal Chemistry, 1990. 4 . Y.H. Sun, L.X. Zhou, and S.Y. Chen et a l . , Acta Petrolei Sinica (Petro Processing section), 6 ( 3 ) ,(1990) 30. 1.
ON TEE ROLE OF REVERSIBLE AND IRREVERSIBLE ADSORPTION HYDROGEN IN THE DEHYDROGENATION AND REFORMING REACTIONS Y. Sun, S. Chen andS. Peng
Instituteof Coal Chemistry,Chinese Academy of Sciences,P.O.Box 165, Taiyuan,Shanxi 030001, China
The conversion of hydrocarbon over Pt catalysts is very important in petroleum processing. Their catalytic performance, particularly product selectivity, depends strongly on the pressure of H2 and its sorption behavior. Thus, much attention has been paid to investigate the effect of H2 adsorption on hydrocarbon conversion over supported Pt catalysts[l] . However, only irreversible adsorption hydrogen has been considered in almost all conclusions. Under a real reaction condition, in our opinion, irreversible and reversible adsorption species co-exist on catalyst surface. The phenomenon has been observed in our experiments[l-3]. Furthermore, it is found that the irreversible and reversible species could play different roles in a catalytic reaction. The reversible adsorption species refers to a type of adsorbed species existing on the catalyst surface at reaction temperature when there is its gas phase pressure in the system and once the pressure is removed by evacuation or purging. it would be completely desorbed, while the irreversible adsorption species can not be removed from the catalyst surface. Therefore, it is necessary t o investigate deeply the roles of the two adsorbed species. The present paper is fucosed on the roles of irreversible and reversible adsorbed hydrogen in dehydrogenation and reforming reaction over supported Pt catalysts.
Catalysts containing 0.5%(wt) platinum were prepared by impregnation of
11 -A1203 of surface area of 239m2/g in HzPtCls solution. The composition
and pretreatment of catalysts were given in Table 1. All catalysts were reduced at 450.c in flowing H2. Both static chemisorption by Chemisorb 2800 (Micromeritics Co., USA) and dynamic adsorption by pulse and FTPS methodl41 was used to measure the hydrogen isobars at different H2 pressure and temperature from 35.c to 550.c. Here the reversible and irreversible uptake of H2 was represented
by Hr/Pt and Hi/Pt respectively, and the fraction of reversibly and irreversibly adsorbed hydrogen in total uptake was symbolized by Fr and Fi respectively. Table 1 Composition and pretreatment conditions catalysts Ga/Pt atom ratio Cl/Pt atom ratio PH value of soiution Calcination temp. (%I
AH1
AH4 ---- AH21 ---- AH2 --- ----
G11 1.04 1.41 2.28 1.32 1.24 2.17 2.54 1.94 1.94 1.15 2.54 450 120 450 450 450
GP2 CP3 2.08 2.89 1.72 ---2.54 2.54 450 450
GP4 3.78 1.46 2.54 450
The data of the coversion of n-hexane at 360.c and 450'c and the dehydrogenation of cyclohexane at 300% in the pressure of H2 were determined with 150 mg of catalyst in a continuous flow microreactor. The feed rate of H2 and n-hexane with volume ratio 1OOO:l was 60 ml/min, and for dehydrogenation, the mole ratio of Hz and cyclohexane is 1O:l. The reliable data were taken after reacting 10 hours. The selectivity of isomerization, dehydrocycl ization and hydrocracking was represented by Si, Sb and Sh respectively. The deactivation amount was given by the difference AX of conversions at initial time Xo and after running 10 hours Xi00
3.RESULT AND DISCUSSION 3.1.The Effect of the Irreversible Hydrogen on the Dehydrogenation of Cyc 1ohexane At 300 'c and atmospheric pressure, benzene was the unique product of cyclohexane dehydrogenation over supported Pt catalysts, and after catalyst running for ten hours, no deactivation was 0bserved.h this case, it was very interesting to note that there was linear dependence between the conversion of cyclohexane and irreversible uptake of hydrogen Fi, which could be expressed empirically as follows: X=1.89-2.44Fi (1) This equation revealed that the irreversibly adsorbed hydrogen on the catalyst surface at the reaction temperature suppressed the dehydrogenation of cyclohexane, i.e., with the increase of the fraction of irreversible hydrogen, catalyst activity showed a 1 inear decrease. Furthermore, the relationship was independent of catalyst preparation. It could, therefore, be reasoned that although the dehydrogenation could also take place on the modified catalyst surface by the irreversible hydrogen, the rate was slower than that on the surfase of fresh catalyst. 3.2. The Effect of Reversible and Irreversible Hydrogen on Conversion of N-hexane For the reaction of n-hexane over Pt catalyst in the presence of hydrogen. the product .was quite sensitive to the reaction temperature. And
2397 therefore, 360'c and 450'c of temperature were selected to study the isomerization and the aromatization of n-hexane. Surprisingly, it was also found that there existed the linear relationships between the reversible hydrogen and the product selectivities and catalyst deactivation amount. They could be empirically expressed by the following equations: at 360.c Si=2.68Fi-. 8354=1.845-2.68Fr r=O.996 (2) r=o.991 (3) 1202-8.3Hr/Pt at 450.c Sb=l.67Fi-0.0334=1.647-1.67Fr r=O.993 (4) r=O.999 (5) SitSh=2.17Fr-1.0472 (6) r=O.996 32-15.OHr/Pt It could be seen from these equations that adsorption type had a crucial effect on the product distribution of n-hexane conversion over supported Pt catalyst, i.e., the increase in the relative amount of irreversible hydrogen promoted the isomerization at 360.c and the aromatization at 450*c,while that in the relative amount of reversible hydrogen did the hydrocracking and isomerization at 450.c ; meanwhile, the reversible hydrogen could protect the deactivation of catalysts. These results demonstrate that not only irreversible but also reversible adsorption species did play important roles in a heterogeneous catalytic reaction, and further indicate that the reaction could take place on the catalyst surface modified by the irreversibly adsorbed hydrogen. This implied that the irreversible species were not only an active reactant but also a modifier of catalyst surface and even could induce reaction or active catalylic sites, while the reversible species mainly act as an active reactant or/and precursor of irreversible species. Furthermore, the negative intercept in equations (2), (4) and (5) implied that the reaction could occur only when there was certcain relative amount of irreversible hydrogen on the surface of the catalyst.and the negative slope in equation (3) and (6) suggested that the supported Pt catalyst would be not deactivated if there was sufficient amount of reversibly adsorbed hydogen on the surface of catalysts.
nX=O.
nX=O.
4. CONCLUSIONS
Not only irreversibly but also reversibly adsorbed hydrogen at the reaction temperature plays important roles in catalytic conversian of hydrocarbon over supported Pt catalyst. The irreversible hydrogen could also induce the reaction o r even modify activate catalytic sites, while the reversible hydrogen acts an active reactant or/and precursor of irreversible species.
The financial support of the National Natural Science Foundation of China is gratefully acknowledged.
2398
REFERENCES: Z. Paol. in Hydrogen Effect in Catalysis - Fund. Prac. Appl(Z. Paol and P.C.Menon, Eds.1 , Marcel Dekker Inc. New York, 1988. 2 . Y.H. Sun, Study on the Influence of Preparation Parameters on the Sorption of H2 and Catalytic Comersion of Cs Hydrocarbons over Supported Pt catalyst, Ph. Dr. Thesis, Institute of Coal Chemistry, 1989. 3 . G.M. Lou, Study on the Effect of Reversibly and Hydrogen on Selective Hydrogenation of Unsaturated Hydrocarbon, Ph. Dr. Thesis, Institute of Coal Chemistry, 1990. 4 . Y.H. Sun, L.X. Zhou, and S.Y. Chen et a l . , Acta Petrolei Sinica (Petro Processing section), 6 ( 3 ) ,(1990) 30. 1.